Previously, pre-stressed concrete pipe and members used for building columns, liquid storage tanks, bridges, fluid flow, etc., have utilized tensioning members made of steel, usually bars or wires commonly called tendons that have been tensioned by stretching with hydraulic jacks, or the like. The goal is to eliminate tension forces on load bearing members, as concrete has considerable compression potential, but little tensile strength. It has been found that many approaches have been attempted in order to solve this problem, including both pre-tensioning and post-tensioning, which are well known in the art, also some have encased concrete pipe in an enclosure and filled the gap with grout or some other solidifying substance under pressure.
A search of the prior art did not disclose any patents that read directly on the claims of the instant invention, however, the following U.S. patents are considered related:
______________________________________ Patent No. Inventor Issue Date ______________________________________ 5,065,795 Creedon Nov. 19, 1991 4,840,200 Creedon Jun. 20, 1989 4,771,530 Creedon Sep. 20, 1988 ______________________________________
U.S. Pat. No. 5,065,795, issued to Creedon, discloses a pre-stressed concrete article that overlays the ends and utilizes gasket rings between the end walls. A hardenable medium is injected in between a jacket and the concrete article which generates sufficient pressure to outwardly bulge the jacket, drawing the end walls together, thereby compressing the gaskets.
Creedon's U.S. Pat. No. 4,840,200 teaches a pre-stressed tube and a method of manufacture. A non-adhering lamina layer receives a medium that is injected under pressure. The medium provides a force to the tube with one or both walls of the layer enlarging and when hardened, provides a permanent leak-proof pre-stress force to the tube. The tube is preferably wound with filaments around a thin walled inner shell and the non-adhering lamina is a film of Teflon or waxed paper. The utility is for lightweight tubes capable of withstanding large bursting forces, such as a rail gun barrel.
U.S. Pat. No. 4,771,530 of Creedon is for methods of pre-stressed concrete members using a pressure containing casing positioned around the outside surface leaving a cavity therebetween. A pressurized medium is injected into the cavity at a pressure sufficient to apply the required pre-stress force. The pressurized medium changes form by hardening after injection and includes grout, thermoset plastic, and epoxy resin. When the concrete member is cast, a cavity is formed by peeling the concrete member and casing away from each other as the medium is injected.
It will be noted that the teachings of pre-stressing a pipe using a hardenable medium under pressure is old and well known in the art, however, simple and reliable methods have been lacking. Further utilization of mechanical features that provide a substantial seal at the end of the tube along with convenient entry points have not been taught by the prior art disclosed.